This invention relates in general to crystal polystyrenes produced by suspension polymerization, and more particularly to a method of improving the behavior of said polystyrenes in injection molding by employing two distinct distributions of molecular weight.
It is known that in order to improve the behavior of polystyrenes produced by suspension polymerization on injection molding, a molecular weight distribution is desirable that is broader than usually obtained by suspension polymerization. The presence of low molecular weight species improves the rate of melt flow at low temperature, while high molecular weight constituents are necessary to obtain adequate strength and to avoid excessive brittleness. Polymers of broad molecular weight distribution tend to have melt flow rates highly sensitive to shear stress and to have broad ranges of moldability (both temperature and pressure).
Polymers of broad and typically single-peaked molecular weight distribution with a number average molecular weight in the range 60,000 to 75,000 and weight average molecular weight of 200,000 to 300,000 are available commercially but are not made by suspension polymerization, a process which offers considerable advantages in simplicity and flexibility. Low molecular weight suspension polymers are made, but do not have the broad molecular weight distribution desired.
It is therefore the object of my invention to broaden the molecular weight distribution for polystyrene.
Another object of my invention is to provide polystyrenes with improved characteristics for injection molding without undue sacrifice of physical properties.
A further object of my invention is to provide a broad molecular weight distribution in polystyrenes made by suspension polymerization.
These and other objects and advantages of my invention will become more apparent from the following detailed description.